That sounds like a plan; for me, four nights in one safari destination is quite a lot...by day three, you will be sleeping in and missing drives, so you may as well be anywhere (well, that's my opinion; YMMV).

Re Stellenbosch, I gave an example itinerary in this thread of how you could do a cool one day itinerary in Stellenbosch. There's also info in that thread on Tintswalo, which is another decent Cape Town option on the water if you don't need to be centrally located.

Mauritius or Mozambique: Mauritius is nice, good family destination especially, but lacks the real wow factor. Mozambique is by far the better shout if the connections work, and if you have a sense of adventure. Re specific places, can't say much as so much has changed in the last few years, but I'm sure others can chime in.